It is generally known that a large magnetizing inrush current flows when non-load magnetizing is performed by supplying power while a transformer core has a residual magnetic flux. The magnitude of this magnetizing inrush current is several times larger than the rated load current of the transformer. The system voltage fluctuates if a large magnetizing inrush current like this flows. If this voltage fluctuation is large, it may have affect customers.
As a method of suppressing this magnetizing inrush current, there is a method of controlling a close phase based on the residual magnetic flux. For example, a method is disclosed by which one arbitrary phase is first closed when turning on a directly-grounded three-phase transformer by using three single-phase circuit-breakers, and then two remaining phases are closed, thereby suppressing a magnetizing inrush current. Additionally, a method which suppresses a magnetizing inrush current when powering on a non-effectively grounded transformer by using a three-phase integrated type circuit-breaker is also disclosed.
Unfortunately, there is no known method which suppresses a magnetizing inrush current generated when opening or closing a circuit-breaker while a transformer is connected to a load. In addition, since a magnetizing inrush current and load current flow through a circuit-breaker while a transformer is connected to a load, a residual magnetic flux after shutdown differs from that when a non-load transformer is shut down.